A bandgap type voltage reference is based on summation of two voltage components having opposite temperature variations. A first component is a base-emitter voltage of a bipolar transistor. This voltage decreases as temperature increases, and therefore may be referred to as a complementary to absolute temperature (“CTAT”) voltage. If a base-emitter voltage is extrapolated back from room temperature close to absolute zero this voltage approaches a constant, referred to as the extrapolated bandgap voltage, denoted Eg0, of the order of 1.15 V to 1.2 V. As temperature increases the base-emitter voltage decreases, and at room temperature the base-emitter voltage is of the order of 600 mV to 700 mV, depending on silicon parameters and bias current. This temperature variation is usually compensated for by using a second voltage component, which is referred to as a proportional to absolute temperature, (“PTAT”). This second voltage component corresponds to a base-emitter voltage difference of two bipolar transistors operating at two different collector current densities.
When the two voltage components, CTAT and PTAT, are well balanced, a compound voltage based on summation still has a second order temperature nonlinearity, referred to as curvature. When this second order error is compensated for, the resulting voltage is said to be temperature insensitive, acting as a voltage reference to Eg0. This creates the undesirable effect for the bandgap type voltage reference that Eg0 is process dependent, differing slightly from process to process, lot to lot, and die to die.
FIG. 1 illustrates a typical process independent voltage reference according to previous configurations. The main objective of this architecture is to be independent of any process variation. In order to achieve this, a target voltage is set that differs from Eg0. This target voltage can represent a base-emitter voltage at a given temperature, such as around ambient. This voltage is sensitive to process and bias conditions, but can be measured and adjusted to a given value.
The configuration in FIG. 1 includes an integrated circuit including a bipolar transistor Q1, here assumed to be a substrate bipolar transistor, biased with a current I1 from a current source. The integrated circuit also includes an amplifier A1, two switches S1 and S2, a second bias current I2 from a second current source, and a feedback resistor, Rf. In a normal operating mode S1 is closed and S2 is open. As a result, the output voltage, the voltage at the amplifier's output node, consists of three added voltage components: a base-emitter voltage of Q1, an amplifier offset voltage, and a voltage drop across Rf due to the bias current I2. The configuration in FIG. 1 assumes that the second order errors such as “curvature” are zero and the voltages are linearly related to absolute temperature.
The voltage reference in the configuration in FIG. 1 is trimmed at two temperatures such that the two trimmings do not interfere with each other. This can be accomplished by forcing bias current I2 to zero at a first temperature, T1, which means that it has a temperature dependency such that it is extracted from the amplifier A1's inverting node for temperatures below T1, and is injected to the inverting node for temperatures higher than T1. Bias current I2 corresponds to a difference of two currents, one current corresponding to the PTAT and one current corresponding to CTAT. At a temperature where T=T1, S1 can be opened and S2 is closed with the bias current I2 forced to zero. In this situation, the output voltage results in the base-emitter voltage of Q1 plus an offset voltage of the amplifier. This output voltage however, is process dependent. In order to compensate for process variation at this first temperature, the bias current I1 is trimmed in such a manner that the output voltage always remains the same. At a second temperature, T2, S1 is closed and S2 is open. At this second temperature, the feedback resistor Rf is trimmed to force the output voltage to the same voltage that was present after the initial trimming step. As a result, the output voltage maintains the same value at the two temperatures, T1 and T2, and is temperature independent. A significant drawback of the configuration of the voltage reference configuration of FIG. 1, however, is the excessively large trimming range of the bias current I2 that is required to cover all process variations.
Thus there remains a need in the art, for a voltage reference circuit that has an improved bandgap voltage without a large trimming range. There further remains a need in the art for an improved temperature coefficient spread for only a single temperature trim.